Procollagen C-proteinases (PCPs) cleave the C-propetides of procollagens Mil, to yield monomers that form the major fibrous components of ECM. The two PCP enhancer proteins (PCOLCE1 and 2) are not proteinases, yet they somehow enhance PCP activities approximately 10-fold in vitro. The PCOLCE1 C-terminal domain can inhibit matrix metalloproteinases (MMPs) secreted by tumor cells, and provides an anti-angiogenic activity isolated from cartilage. PCOLCE1 may also play a role in growth control, as disruption of the cognate gene results in anchorage-independent growth in cultured fibroblasts. We have generated mice with null alleles for the PCOLCE1 gene Pcolce, to ascertain its in vivo roles. Pcolce-/- mice are viable and fertile, but have profound ECM perturbations in bone and soft tissues, thus identifying PCOLCE1 as a determinant of bone strength and ECM integrity. More recently, we have succeeded in generating mice with null-alleles for the PCOLCE2 gene, Pcolce2, and have found Pcolce2-/- mice to be both viable and fertile. Predominant developmental expression of Pcolce2 in cartilage, plus the skeletal phenotype of Pcolce-/- mice, suggest role(s) for Pcolce2 in endochondral bone formation and in pathogenesis of arthridities. Both Pcolce and Pcolce2 are notable for particularly high expression levels in heart and Pcolce2 is also expressed at high levels in eye. We propose fully characterizing adult and embryo Pcolce-/- and Pcolce2-/- mice and derived cells, to determine the molecular bases and full extents of skeletal and soft tissue abnomalities. Crosses of Pcolce-/- and Pcolce2-/- mice to produce Pcolce/Pcolce2 doubly homozygous null progeny will remove any residual activity due to possible functional redundancy between Pcolce and Pcolce2. Studies of Pcolce-/-, Pcolce2-/- and doubly null mice, embryos and cells will enable clear delineation of PCPE roles in mammalian development, homeostasis and pathology. Based on recent evidience in our lab of physical interactions between PCOLCE1 and BMP1-like PCPs, we also propose studies that promise a paradigmatic shift in our understanding of how PCPs and PCOLCEs function.